Punching or blanking operations, particularly for the mass-production of metallic components made of metal plate, for example for the automotive sector or for the electrical household appliance sector, generally use presses to the sliders of which one or more punching devices with plate pressers are applied, while corresponding dies are coupled to the fixed part of such presses; the metal plate to be worked is rested on such dies, and the punches, by descending, enter such dies in order to remove material.
A punching device with plate presser generally comprises a tubular containment body, which is closed by a bottom at one end and by a head at the opposite end; such head is perforated for the passage of a piston-operated plate presser, which performs a translational motion within such body and is crossed coaxially by a punch, a containment chamber for elastic pusher means for the plate presser is formed between such plate presser and such bottom.
In known punching devices with plate pressers, the elastic means are constituted either by a metallic helical spring or by an elastomeric element made of plastics.
Although these devices are widespread, they have drawbacks, the first of which is a pressing force generated by the plate presser on the metal plate that is insufficient to work so-called high-strength metal plates, i.e., with a very low thickness (for example between 0.2 and 0.5 mm) and made of high-strength steels, characterized by a tensile strength that is triple that of ordinary structural steels, and with mechanical properties that depend on phosphorus and manganese silicates, which are present in small quantities, and on the extremely low carbon content.
Helical springs, like elastomeric elements, in fact have a substantially linear compression curve, according to which the thrust force is zero if compression is nil; therefore, as the compressed helical spring (or elastomeric element) that pushes against the plate presser extends, i.e., when the punching device is lifted from the metal plate that it has just worked, the pressing force on the plate decreases, and does so in the most delicate situation, when the punch, lifted by the slider of the press, is made to exit from the punched metal plate, dragging with it the edges of the generated hole, with consequent unwanted deformation of such edge with respect to the desired flatness.
It is instead in this step that the plate presser in fact should express the maximum pressing force.
Further, in order to modify the pressing force of a plate presser, it is only possible to intervene by changing the entire punching device, since the tubular bodies of the various punching devices are designed to contain helical springs or elastomeric elements of very specific dimensions, which when compressed undergo deformations (i.e., expand diametrically) and therefore are not preset to contain elastic means of different dimensions to obtain different thrusts.
For elastomeric elements in particular, the compression length is limited, since the diametrical deformation is proportional to such compression length.
But it is the compression length of the spring that determines the maximum stroke of the punch in the metal plate, where the length of the stroke can be decisive in punching metal plates made of steels that are particularly designed to deform plastically and require a longer stroke than usual (for example if a stroke of 12-15 mm is needed but the obtainable stroke is 10 mm).
Further, plate pressers are currently provided which must be fitted on known types of punch holder pressing fixtures, so as to adapt to punches that are already commercially available, and can be fitted on the corresponding punch holding fixtures by way of reversible snap-acting or bayonet engagement means, which also are of the known type.